Is Obama's executive action on guns legal? Let's break it down.

On Tuesday, President Barack Obama announced new executive actions aimed at addressing American gun violence.

The measure that's sure to evoke the most debate -- and which has already done so -- is his plan to extend the use of background checks to cover more sales at gun shows. In 2013, after a strong push from the president, the Senate blocked an effort to expand background checks at gun shows. In lieu of that legislative action, Obama is pushing to reinterpret where the line lies between those who must conduct background checks and those who don't need to.

That distinction centers on the phrase "engaged in the business." Those who are engaged in the business of selling firearms, such as firearm dealers, need to conduct background checks. Those who aren't, such as individuals selling guns, don't.

Under existing law, the difference is codified loosely. A firearm dealer is someone who "devotes time, attention, and labor to dealing in firearms as a regular course of trade or business with the principal objective of livelihood and profit through the repetitive purchase and resale of firearms," but is not someone "who makes occasional sales, exchanges, or purchases of firearms for the enhancement of a personal collection or for a hobby, or who sells all or part of his personal collection of firearms."

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Per the White House, the Bureau of Alcohol, Tobacco, Firearms and Explosives has clarified that you don't need a brick-and-mortar store to be someone who is "engaged in the business" of selling guns -- and that there's no hard number to delineate a dealer from an individual.

"Even a few transactions, when combined with other evidence, can be sufficient to establish that a person is 'engaged in the business,'" the White House memo reads. And as a reminder, you can be fined as much as $250,000 if you are engaged in the firearms business but don't conduct background checks as required.

Because this is inevitably going to become a legal battle, we sought the opinion of Franklin Zimring, a law professor at the University of California at Berkeley and a former director of research for the Task Force on Firearms of the National Commission on the Causes and Prevention of Violence, formed by President Lyndon Johnson.

Here's where we are now: A guy who owns a gun store is a dealer and runs a background check. A guy who wants to sell his one rifle to his neighbor isn't a dealer and doesn't have to conduct one. The space in the middle is big and encompasses a lot of activity -- and is intentionally hazy.

At their core, background checks aim to do two things, Zimring explained. "That is: Make sure that people who are entitled to have guns have all the guns they want, and keep people who aren't entitled to have guns from having any." The goal of the background check is to differentiate between those two groups, but the very existence of the dealer-individual distinction "creates an enormous loophole," in Zimring's phrasing.

Gun shows are the embodiment of that loophole. He imitated a carnival barker. " 'Minors! People with felony records! Step right up!' This is not only a loophole," Zimring said, "but it is a loophole with enough public notification so that it isn't difficult to get the people who don't want to have their backgrounds checked together with the sellers who aren't going to check them."

Dealer and "engaged in the business" are "terms of art," Zimring said. "The legislative enactments that they're organized to express are compromises," attempts to sketch a light line in that murky middle. What Obama is trying to do is to move that line determining who is a dealer, shifting where that compromise has been understood to lie.

"It is certainly consistent with the capacity and responsibility of the executive branch of government to make legislative standards coherent and clear," Zimring said. Obama is "trying to make it coherent and clear -- and give it more reach." The president's recent actions on immigration involve refocusing where law enforcement directs its energy. This executive action, in Zimring's assessment, is less complicated.

"Remember that the border between dealer and individual seller is an arbitrary one," he said. "Can you simply take the number of shows you go to or the number of guns you sell and say that makes you a dealer?" he wondered. "The answer is: Why not?"

The answer to "why not" to this point has been politics. Any president could have been more deliberate about the dealer-individual distinction, but it's politically fraught. Congress has repeatedly shown a lack of interest in moving that line. The very existence of the distinction prompted some bemusement from Zimring. "You don't deliberately create a loophole to a permissive licensing system unless you've been elected to Congress in the United States of America!"

Again, Obama's efforts will undoubtedly, at some point, end up in front of a panel at the Court of Appeals for the District of Columbia Circuit, where it's anyone's guess about what happens. Until that point, some gun show attendees who consider themselves individuals and who don't think they're "engaging in the business" of selling firearms may suddenly find that the ATF disagrees. Others may err on the side of caution, to Obama's approval, and try to implement background checks for their customers.

And that raises a whole other issue: Getting that done. Obama's proposal would expand the FBI's resources for conducting background checks, but that doesn't necessarily make it trivial. "How easy it is to do technologically?" Zimring asked. "That depends on the investment of resources."